1. Field of the Invention
This invention relates to an improvement of a tonometer, in which the reliability on a result of eye pressure measurement is much improved while taking into consideration the eye pressure fluctuation due to pulse fluctuation.
2. Related Art of the Invention
As a conventional tonometer, there is known a noncontact type tonometer, e.g., air puff type tonometer, in which a fluid pulse is discharged toward an eye of a subject to be tested and the pressure value of the eye to be tested is measured based on a corneal applanation of the eye. In this type of a tonometer, an air pulse as the fluid pulse is momentarily discharged towards the eye to be tested in such a short time as several tens ms for example and the cornea is flattened or applanated in such a short time as several tens ms to carry out an eye pressure measurement. In this case, however, there is present the following inconvenience. That is, the pressure of the eye to be tested changes corresponding to the pulse fluctuation. It is said that the eye pressure fluctuation is several mmHg maximum. While the eye pressure value of a normal eye is normally 10 mmHg to 20 mmHg, his pulse fluctuation is normally 60 to 120 times per minute. The cycle T (see FIG. 7) of a pulse wave A of the pulse fluctuation is approximately 500 ms at the shortest. Therefore, if the eye pressure is measured without taking into consideration the eye pressure fluctuation due to pulse fluctuation, the measured value itself becomes dubious. The reason is that if the eye pressure measurment is carried out at the top A.sub.1 of the pulse wave A as shown in FIG. 7, the eye pressure value obtained becomes higher corresponiding thereto, while if the eye pressure measurement is carried out in the bottom A.sub.2 of the pulse wave A, the eye pressure value obtained becomes lower corresponding thereto.
To overcome this inconvenience, there is proposed a tonometer for measuring an eye pressure taking into consideration the eye pressure fluctuation due to pulse fluctuation (U.S. Pat. No. 3,572,100). In the tonometer disclosed in this U.S. Pat. No. 3,572,100, an eye pressure measurement is carried out in synchronism with the same phase place A.sub.3 of the pulse wave A due to pulse fluctuation. According to this tonometer, errors of measurement due to pulse fluctuation can be removed.
In general, it is required for a noncontact type tonometer to ensure a strict alignment with respect to an eye to be tested. In order to carry out the eye pressure measurement correctly, such a strict alignment accuracy is required as a zero point something mm or less. Therefore, in the tonometer disclosed in the U.S. Pat. No. 3,572,100, since the eye pressure measurement is carried out in synchronism with the same phase place A.sub.3 of the pulse wave A due to pulse fluctuation 1 to 2 times per second, the aligning state of the tonometer must be maintained for approximately 1 second maxiumum with respect to the eye to be tested. However, since the eye to be tested repeats a fixation flick or tremor at the cycle of 0.2 to 3 seconds within the range of from zero point something mm to 1 mm, it is difficult to maintain the aligning state for 1 second. Therefore, the inspector or examiner needs a long experience. In addition, the operation of this type of conventional tonometer is troublesome.